The preferred embodiments relate to voltage controlled oscillator (VCO) technology and, more particularly, to a quadrature VCO (QVCO).
A VCO is a device (i.e., oscillator) that outputs an oscillating signal with a frequency that is controlled by the level of an input voltage applied to the VCO. A fixed DC input voltage to the VCO, therefore, should ideally produce a fixed output frequency signal, whereas that input voltage also may be varied so as to vary the VCO output frequency. As to the latter, therefore, a modulating input signal may be applied to cause the VCO to output a signal with a modulating frequency (or phase).
A certain type of modulation scheme that packs more than 1 bit/symbol for communicating data, and relevant to the preferred embodiments is quadrature phase-shift keying (QPSK). In QPSK, a VCO provides quadrature oscillating signals, thereby operating as a QVCO, where the quadrature signals consist of four different oscillating signals, each 90 degrees apart from the others. More specifically, as a type of phase-shift keying (PSK), QPSK communicates data by modulating (i.e., changing) a phase of a carrier signal. The term “quadrature” in QPSK indicates that there are four different phases, each preferably orthogonal with respect to one another, for the data modulation. More specifically, therefore, a single data quantity, or “symbol,” may be represented by any one of the four available phases, typically equispaced in the QPSK constellation at locations that are 90 degrees apart from one another and located at angles π/4, 3π/4, 5π/4 and 7π/4 to the real axis. Each location of these separate locations may represent a different one-of-four combination of two binary bits, thereby permitting communication of a symbol representing a binary value of 11, 01, 00, or 10. In QPSK, such data is modulated, and demodulated, typically by separating a bit stream into two separate bit streams, denoted as an in-phase stream designated as I and a quadrature phase designated as Q. The I data is modulated by a first signal (e.g., sine wave), while the Q data is modulated by a second signal that is 90 degrees apart (e.g., cosine wave) from the first signal, with the results added to provide the transmitted QPSK signal. Demodulation is achieved in a reverse process.
Given the preceding, of note in connection with QPSK methodology is that it involves quadrature phases, and in electronic circuits such phases are typically implemented using a VCO that is locked into quadratures, that is, having four different outputs that, as introduced earlier, are 90 degrees apart from one another. Such an architecture is typically referred to as a quadrature VCO, or abbreviated as QVCO. Further, quadrature local oscillation also may be used and be important in other applications, such as image rejection in receivers. By way of further background, therefore, three prior art QVCOs are described below.
FIG. 1 illustrates a schematic of a prior art source coupled QVCO shown generally at 10 and that, as shown below, obtains its name due to the nature of coupling signals as between respective transistor source nodes. QVCO 10 includes two symmetric oscillating circuits 20 and 40, which are inductively coupled together so as to lock the operation and oscillating signals of the two in quadrature, as will be explained later. Since circuits 20 and 40 are symmetric, the following discussion will detail circuit 20 by way of example, followed by a discussion of the comparable circuit 40.
Oscillating circuit 20 includes a first inductor 22 and a second inductor 24, each having a respective first terminal 22T1 and 24T1 connected to receive a fixed voltage potential, shown as VDD, or note that inductors 22 and 24 may represent a single inductor with VDD connected to the center tap of that inductor. A second terminal 22T2 of inductor 22 is connected to a node 26, which is also connected to a drain of an nMOS transistor 28, and a second terminal 24T2 of inductor 24 is connected to a node 30, which is also connected to a drain of an nMOS transistor 32. The sources of nMOS transistors 28 and 32 are connected to a node 34, and the gates of nMOS transistors 28 and 32 are cross-coupled, that is, the gate of nMOS transistor 28 is connected to the drain of nMOS transistor 32, and the gate of nMOS transistor 32 is connected to the drain of nMOS transistor 28. Node 34 is connected to a first terminal 36T1 of a third inductor 36, and the second terminal 36T2 of third inductor 36 is connected to ground. Lastly, note that node 26 provides a first oscillating output signal I+, and node 30 provides a second oscillating output signal I−, where these two signals are antiphase (i.e., ideally 180 degrees apart) and occur in part in response to an induced signal from oscillating circuit 40, as also further detailed later.
As introduced above, oscillating circuit 40 is comparable in devices and connections with respect to oscillating circuit 20. As now will be appreciated, however, oscillating circuit 40 provides the quadrature Q outputs in part in response to an induced signal from oscillating circuit 20. Specifically, oscillating circuit 40 includes a first inductor 42 and a second inductor 44, each having a respective first terminal 42T1 and 44T1 connected to receive the fixed voltage potential, VDD (or again, those two inductors being a single inductor with VDD at its center tap). A second terminal 42T2 of inductor 42 is connected to a node 46, which is also connected to a drain of an nMOS transistor 48, and a second terminal 44T2 of inductor 44 is connected to a node 50, which is also connected to a drain of an nMOS transistor 52. The sources of nMOS transistors 48 and 52 are connected to a node 54, and the gates of nMOS transistors 48 and 52 are cross-coupled, that is, the gate of nMOS transistor 48 is connected to the drain of nMOS transistor 52, and the gate of nMOS transistor 52 is connected to the drain of nMOS transistor 48. Node 54 is connected to a first terminal 56T1 of a third inductor 56, and the second terminal 56T2 of third inductor 56 is connected to ground. Lastly, note that node 46 provides a first oscillating output signal Q+, and node 50 provides a second oscillating output signal Q−, where these two signals are ideally 180 degrees apart and occur in part in response to an induced signal from oscillating circuit 20, as also further detailed later.
The operation of QVCO 10 is now generally described for context in this document, with additional aspects readily known or ascertainable by one skilled in the art. Looking first to oscillating circuit 20, in general an oscillating loop may be observed with respect to inductors 22 and 24 and nMOS transistors 28 and 32. Particularly, with this loop, energy oscillates between the inductance and inherent transistor capacitance, and while resistance also exists in the circuit that would tend to diminish the response of the circuit, as known in the VCO art the cross-coupling of the gates of nMOS transistors 28 and 32 provides a negative conductance, sometimes also referred to as a −R, so as to compensate for this resistance and to maintain the loop in oscillation. Thus, this loop provides the oscillating signals I+ and I− which, as energy is exchanged in the loop and given the cross-coupling of nMOS transistors 28 and 32, causes the drain of one of those transistors to rise in voltage while the other falls in complementary fashion, and vice versa, such that I+ and I− at those drains are anti-phase signals, that is, they remain 180 degrees (i.e., one half-period) apart from one another. In addition to the above-described loop operation, note also that as one of nodes 26 and 30 rises, the other falls, thereby creating a symmetric and complementary signal in the other node, where these signals are also applied via nMOS transistors 28 and 32 to inductor 36. Inductor 36 then couples this energy to inductor 56 of oscillating circuit 40, that is, inductor 36, coupled to the sources of nMOS transistors 28 and 32, is coupled as one coil in a two coil transformer to inductor 56 of oscillating circuit 40, thereby coupling to the sources of nMOS transistors 48 and 52. Thus, the coupling of nMOS transistor 28 and 32 sources to nMOS transistor 48 and 52 sources gives rise to the descriptor of QVCO 10 being a source coupled device. Moreover, the polarity of the coupling between inductors 36 and 56 is shown by the known dot convention in the illustration, and, therefore, a current in one direction relative to inductor 36 corresponds to a current in the opposite direction in inductor 56. Thus, as further appreciated below, inductors 36 and 56 effectively form a transformer, whereby the antiphase signals of one loop are injected into the other, and the reverse currents of the two coupled inductors maintain an opposing-current and voltage and phase-shifted synchronization between one another.
Looking to oscillating circuit 40, it generally operates in the same way as oscillating circuit 20, albeit in a phase-shifted fashion, and the reader is assumed familiar therefore with the previous discussion which is now briefly summarized with respect to the comparable circuit 40. Generally, the loop of inductors 42 and 44 and nMOS transistors 48 and 52 oscillates and the opposing complementary potential at the drains of those transistors provides the antiphase output pairing of Q+ and Q−. Further, that loop is modulated by the coupling of the signals I+ and I−, from circuit 20 and via its inductor 36, into the respective inductor 56 in oscillating circuit 40.
Having described the general operation of circuits 20 and 40, note further that such operation locks oscillating circuit 20 in quadrature with respect to oscillating circuit 40. As noted above, in each circuit the outputs (i.e., either Q+ and Q− or I+ and I−) are antiphase signals. Particularly, oscillating circuit 20 includes inductor 36, and oscillating circuit 40 includes inductor 56, and the inductance of each may be selected so that the frequency of the common source node (i.e., node 34 or node 54) will resonate at the twice the frequency of the respective antiphase signals of each circuit, thereby maintaining the source voltage at all time below the drain voltages for each circuit; thus, in circuit 20, the voltage at node 34 remains below that of either nodes 26 and 30, and in circuit 40, the voltage at node 54 remains below that of either nodes 46 and 50. Moreover, with sufficient current through each of inductors 36 and 56, the loop non-linearity will select as between the odd mode and even mode, selecting that which has the higher oscillation amplitude. In this regard, the odd mode will prevail, thereby locking the two oscillating circuits in quadrature with respect to one another. The electromagnetic coupling as between inductors 36 and 56, as shown by the dot convention, further ensures the odd mode in that, as the dots indicate, a negative electromagnetically coupling factor (i.e., current in opposing directions) is maintained which, with the chosen values of inductors 36 and 38, maintains the common source voltages at nodes 34 and 54 antiphase with respect to one another (e.g., at twice the overall oscillation frequency), thereby locking the circuits in the odd mode and locking the output nodes in quadrature. Thus, as any one of the in-phase or quadrature nodes (26, 30, 46, or 50) varies, each of the others will vary and remain locked in quadrature, that is, maintaining each of the four signals at 90 degrees apart from one another.
FIG. 2 illustrates a schematic of a prior art drain-gate coupled QVCO shown generally at 60 and that, as shown below, obtains its name due to the nature of coupling signals as between a drain in one oscillating circuit and a transistor gate in another oscillating circuit. QVCO 60 includes bias control circuitry 62 that may be constructed according to known principles for biasing a QVCO and more particularly for biasing QVCO 60, as further explored below. QVCO 60 also includes two symmetric oscillating circuits 70 and 100, which are inductively coupled together so as to lock the operation and oscillating signals of the two in quadrature, as will be explained later. Since circuits 70 and 100 are symmetric, the following discussion will detail circuit 70 by way of example, followed by an overview of the comparable circuit 100.
Oscillating circuit 70 includes a first inductor 72 and a second inductor 74, each having a respective first terminal 72T1 and 74T1 connected to a first terminal of a resistor R1, where the second terminal of resistor R1 is connected to receive a fixed voltage potential, shown as VDD. A second terminal 72T2 of inductor 72 is connected to a node 76, which is also connected to a drain of an nMOS transistor 78, and a second terminal 74T2 of inductor 74 is connected to a node 80, which is also connected to a drain of an nMOS transistor 82. The source of nMOS transistor 78 and of nMOS transistor 82 is connected to a node 84. The gate of nMOS transistor 78 is connected to a first terminal 86T1 of an inductor 86, and the gate of nMOS transistor 82 is connected to a first terminal 88T1 of an inductor 88. The second terminal 86T2 and the second terminal 88T2 of inductors 86 and 88, respectively, are connected together and to a first terminal of a resistor R2, which has its second terminal connected to terminals 72T1 and 74T1 of inductors 72 and 74. Node 84 is connected to a drain of an nMOS transistor 90, which has it source connected to ground and its gate connected to bias control circuitry 62. Lastly, note that node 76 provides a first oscillating output signal I+, and node 80 provides a second oscillating output signal I−, where these two signals are ideally 180 degrees apart and occur in part in response to an induced signal from oscillating circuit 100, as also further detailed later.
As introduced above, oscillating circuit 100 is comparable in devices and connections with respect to oscillating circuit 70. Oscillating circuit 100, however, provides the quadrature Q outputs and operates in response to induced signals from I+ and I− of oscillating circuit 70. Specifically, oscillating circuit 100 includes a first inductor 102 and a second inductor 104, each having a respective first terminal 102T1 and 104T1 connected to a first terminal of a resistor R3, where the second terminal of resistor R3 is connected to VDD. A second terminal 102T2 of inductor 102 is connected to a node 106, which is also connected to a drain of an nMOS transistor 108, and a second terminal 104T2 of inductor 104 is connected to a node 110, which is also connected to a drain of an nMOS transistor 112. The source of nMOS transistor 108 and of nMOS transistor 112 is connected to a node 114. The gate of nMOS transistor 108 is connected to a first terminal 116T1 of an inductor 116, and the gate of nMOS transistor 112 is connected to a first terminal 118T1 of an inductor 118. The second terminal 116T2 and the second terminal 118T2 of inductors 116 and 118, respectively, are connected together and to a first terminal of a resistor R4, which has its second terminal connected to terminals 102T1 and 104T1 of inductors 102 and 104. Node 114 is connected to a drain of an nMOS transistor 120, which has it source connected to ground and its gate connected to bias control circuitry 62. Lastly, note that node 106 provides a first oscillating output signal Q+, and node 110 provides a second oscillating output signal Q−, where these two signals are ideally 180 degrees apart and occur in part in response to an induced signal from oscillating circuit 70, as also further detailed later.
Oscillating circuits 70 and 100 also are inductively coupled to one another, as is now described. The couplings in this respect are shown in FIG. 2 using the known inductor dot convention. Thus, inductor 72 of circuit 70 is coupled to inductor 116 of circuit 100, and inductor 74 of circuit 70 is coupled to inductor 118 of circuit 100—this coupling is also shown by way of a dashed inductive path arrow IP1. Hence, the drain of nMOS transistor 78 is coupled via inductor 72 and inductor 116 to the gate of nMOS transistor 108, and the drain of nMOS transistor 82 is coupled via inductor 74 and inductor 118 to the gate of nMOS transistor 112. In this sense, therefore, the pair of inductors 72 and 74 forms a transformer pair with the pair of inductors 116 and 118. Further, inductor 86 of circuit 70 is coupled to inductor 104 of circuit 100, and inductor 88 of circuit 70 is coupled to inductor 102 of circuit 100—this coupling is also shown by way of a dashed inductive path arrow IP2. Hence, the drain of nMOS transistor 108 is coupled via inductor 102 and inductor 88 to the gate of nMOS transistor 82, and the drain of nMOS transistor 112 is coupled via inductor 104 and inductor 86 to the gate of nMOS transistor 78. In this sense, therefore, the pair of inductors 86 and 88 forms a transformer pair with the pair of inductors 104 and 102. As is well known in the transformer art, therefore, a current through one (or one series pair) inductor will induce a current in the inductor to which it is inductively or magnetically coupled, typically by locating the inductors within a proximity to one another, with the distance of this proximity or other—magnetically coupling technique being ascertainable by one skilled in the art.
The operation of QVCO 60 is now generally described for context in this document, with additional aspects readily known or ascertainable by one skilled in the art and also as generally understood from the earlier discussion of FIG. 1. In general, QVCO 60 is a two stage ring oscillator with a coupled resonator as the load providing for a 90 degree phase shift. Looking then in more detail and first to oscillating circuit 70, in general an oscillating loop may be observed with respect to inductors 72 and 74, nMOS transistors 78 and 82, and nMOS transistor 90. Specifically, bias control circuitry 62 maintains nMOS transistor 90 on, and with the draw of current through inductors 72 and 74 an LC loop is created as between those inductors and the intrinsic capacitors of the transistors; moreover, a coupling is achieved in that Q+ and Q− from circuit 100 are coupled into circuit 70, inducing counterpart signals into the gates of transistors 78 and 82 to generate a mutual inductance effect, thereby locking the I+ and I− antiphase signals of circuit 70 into a quadrature spacing with the Q+ and Q− from circuit 100. At the same time, a comparable operation occurs in oscillating circuit 100, whereby inductors 102 and 104 provide an LC loop with nMOS transistors 108 and 112 so as to provide antiphase signals Q+ and Q−, while a cross-coupling is achieved in that I+ and I− from circuit 70 are coupled into circuit 100, inducing counterpart signals into the gates of transistors 108 and 112 to generate a mutual inductance effect, thereby locking the Q+ and Q− antiphase signals of circuit 100 into a quadrature spacing with the I+ and I− from circuit 70. Lastly, note that resistor R2 and resistor R4, of the same resistance, have resistance sufficiently large so as to eliminate any common mode oscillations, while resistor R1 and resistor R3, of the same resistance, have resistance for amplitude control (and may be digitally switched resistors).
FIG. 3 illustrates a schematic of a prior art drain-source coupled QVCO shown generally at 130 and that, as shown below, obtains its name due to the nature of coupling signals as between a transistor drain in one oscillating circuit and a transistor source in another oscillating circuit. QVCO 130 includes bias control circuitry 132 that may be constructed according to known principles for biasing a QVCO and more particularly for biasing QVCO 130, as further explored below. QVCO 130 also includes two symmetric oscillating circuits 140 and 170, which are inductively coupled together so as to lock the operation and oscillating signals of the two in quadrature, as will be explained later. Since circuits 140 and 170 are symmetric, the following discussion will detail circuit 140 by way of example, followed by an overview of the comparable circuit 170.
Oscillating circuit 140 includes a first inductor 142 and a second inductor 144, each having a respective first terminal 142T1 and 144T1 connected to receive a fixed voltage potential, shown as VDD. A second terminal 142T2 of inductor 142 is connected to a node 146, which is also connected to a drain of an nMOS transistor 148, and a second terminal 144T2 of inductor 144 is connected to a node 150, which is connected to a drain of an nMOS transistor 152. nMOS transistors 148 and 152 are cross-coupled, that is, the drain of nMOS transistor 148 is connected to the gate of nMOS transistor 152, and the drain of nMOS transistor 152 is connected to the gate of nMOS transistor 148. The source of nMOS transistor 148 is connected to a terminal 154T1 of an inductor 154, and the source of nMOS transistor 152 is connected to a terminal 156T1 of an inductor 156. A second terminal 154T2 of inductor 154 and a second terminal 156T2 of inductor 156 are connected to a drain of an nMOS transistor 158, which has its source connected to ground and its gate connected to bias control circuitry 132. Lastly, note that node 146 provides a first oscillating output signal I+, and node 150 provides a second oscillating output signal I−, where these two signals are ideally 180 degrees apart and occur in part in response to an induced signal from oscillating circuit 170, as also further detailed later.
As introduced above, oscillating circuit 170 is comparable in devices and connections with respect to oscillating circuit 140. Oscillating circuit 170, however, provides the quadrature Q outputs and operates in response to induced signals from I+ and I− of oscillating circuit 140. Specifically, oscillating circuit 170 includes a first inductor 172 and a second inductor 174, each having a respective first terminal 172T1 and 174T1 connected to receive VDD. A second terminal 172T2 of inductor 172 is connected to a node 176, which is also connected to a drain of an nMOS transistor 178, and a second terminal 174T2 of inductor 174 is connected to a node 180, which is connected to a drain of an nMOS transistor 182. nMOS transistors 178 and 182 are cross-coupled, that is, the drain of nMOS transistor 178 is connected to the gate of nMOS transistor 182, and the drain of nMOS transistor 178 is connected to the gate of nMOS transistor 178. The source of nMOS transistor 178 is connected to a terminal 184T1 of an inductor 184, and the source of nMOS transistor 182 is connected to a terminal 186T1 of an inductor 186. A second terminal 184T2 of inductor 184 and a second terminal 186T2 of inductor 186 are connected to a drain of an nMOS transistor 188, which has its source connected to ground and its gate connected to bias control circuitry 132. Lastly, note that node 176 provides a first oscillating output signal Q+, and node 180 provides a second oscillating output signal Q−, where these two signals are ideally 180 degrees apart and occur in part in response to an induced signal from oscillating circuit 140, as also further detailed below.
Oscillating circuits 140 and 170 also are inductively coupled to one another, as is now described. The couplings in this respect are shown in FIG. 3 using inductor dot convention. Thus, inductor 142 of circuit 140 is coupled to inductor 184 of circuit 170, and inductor 144 of circuit 140 is coupled to inductor 186 of circuit 170—this coupling is also shown by way of a dashed inductive path arrow IP1. Hence, the drain of nMOS transistor 148 is coupled via inductor 142 and inductor 172 to the source of nMOS transistor 178, and the drain of nMOS transistor 152 is coupled via inductor 144 and inductor 186 to the source of nMOS transistor 182. In this sense, therefore, the pair of inductors 142 and 144 form a transformer pair with the pair of inductors 184 and 186. Further, inductor 172 of circuit 170 is coupled to inductor 156 of circuit 140, and inductor 174 of circuit 170 is coupled to inductor 154 of circuit 140—this coupling is also shown by way of a dashed inductive path arrow IP2. Hence, the drain of nMOS transistor 178 is coupled via inductor 172 and inductor 156 to the source of nMOS transistor 152, and the drain of nMOS transistor 182 is coupled via inductor 174 and inductor 154 to the source of nMOS transistor 148. In this sense, therefore, the pair of inductors 172 and 174 form a transformer pair with the pair of inductors 156 and 154. As is well known in the transformer art, therefore, a current through one (or one series pair) inductor will induce a current in the inductor to which it is inductively or magnetically coupled, typically by locating the inductors within a proximity to one another, with the distance of this proximity or other magnetically coupling technique being ascertainable by one skilled in the art.
The operation of QVCO 130 is now generally described for context in this document, with additional aspects readily known or ascertainable by one skilled in the art and also as generally understood from the earlier discussion of FIGS. 1 and 2. Looking first to oscillating circuit 140, in general an oscillating loop may be observed with respect to inductors 142 and 144, nMOS transistors 148 and 152, inductors 154 and 156, and nMOS transistor 158. Specifically, bias control circuitry 132 maintains nMOS transistor 158 on, and with the draw of current through inductors 142 and 144 and 154 and 156, an LC loop is created as between those inductors and the intrinsic capacitors of the transistors along with the negative conductance provided by the cross-coupling of the gates of nMOS transistors 148 and 152; moreover, a coupling is achieved in that Q+ and Q− from circuit 170 are coupled into the loop of circuit 140 to generate a mutual inductance effect, thereby locking the I+ and I− antiphase signals of circuit 140 into a quadrature spacing with the Q+ and Q− from circuit 170. At the same time, a comparable operation occurs in oscillating circuit 170, whereby inductors 172 and 174, and inductors 184 and 186, provide an LC loop with nMOS transistors 178 and 182, so as to provide antiphase signals Q+ and Q−, while a coupling is achieved in that I+ and I− from circuit 140 are coupled into circuit 170 to generate a mutual inductance effect, thereby locking the Q+ and Q− antiphase signals of circuit 170 into a quadrature spacing with the I+ and I− from circuit 140.
While the above and related approaches have served various needs in the prior art, they also provide various drawbacks. For example, the use of inductors in the prior art can account for a large percentage of the total area needed to implement the circuit, such that an amount of approximately seventy percent of the overall device layout is consumed by the device inductors. Moreover, power consumption also is a concern with QVCO devices.
Given the preceding, the present inventors seek to improve upon the prior art, as further detailed below.